1. Field of the Invention:
This invention pertains to a remedial treatment of coal degas wells to enhance the production of gas and/or water from the wells. An aqueous foam is used as the well treatment fluid.
2. Description of the Prior Art:
Coal is the most abundant fossil energy resource in the world. Its recoverable reserves amount to almost 100 quintillion Btu of energy, nearly 15 times the total energy content estimated for known reserves of petroleum. Petroleum Frontiers, Vol. 3, No. 4, pages 2-3 (1986), published by Petroleum Information Corporation. People have mined coal and used it for heat for centuries. However, it is within the recent past that coal has been recognized for being the origin and source for another hydrocarbon fuel, i.e., coalbed methane. Coalbed gas consists primarily of methane (e.g., 95 percent) but may also contain ethane, propane and higher homologs. The volume of coalbed methane is estimated to be about 400 trillion standard cubic feet (SCF) of gas-in-place most of it adsorbed on coal seams buried at a depth of less than 9000 feet (ft) from the surface, and almost half of it is a coal seam buried less than 3000 ft. Too deep to mine but easily penetrated by a wellbore using conventional drilling techniques. Coalbeds are, therefore, reservoirs and source rocks for a huge amount of gas which can be produced, in part, through a wellbore. Methods of recovering the gas (i.e., coal degasification methods) are shown, for example, by U.S. Pat. Nos. 4,471,840, 4,391,327 and 4,301,875.
Much work has been done to capture the prize. The U.S. Department of Energy, the U.S. Bureau of Mines and the Gas Research Institute have funded a substantial amount of research on coal degasification, and the results have been published in the open literature. In addition, periodic coalbed methane symposiums are held at the University of Alabama, and elsewhere, and the results are published as symposium proceedings. Many of the journal articles describe stimulation techniques used by the industry to enhance production of gas. Conventional hydraulic fracturing techniques are the most common. In hydraulic fracturing, a fracture fluid (e.g., an aqueous gel or an aqueous foam) is injected through a wellbore and against the face of the formation at pump rates and pressure sufficient to fracture the formation. Typically, a proppant (e.g., 20/40 mesh sand, bauxite and the like) is blended with the fracture fluid and is carried by the fluid into the fracture. When the pump rate and pressure are released, the fractured formation settles back onto the emplaced proppant and a permeable communication channel is thereby established from the tip of the pack of proppant back to the wellbore. The formation fluids flow through this communication channel to the wellbore and are withdrawn. Unfortunately, coal fines are simultaneously carried towards the wellbore by the produced fluids and, occasionally, the communication channel is plugged by the coal fines. The plugging generally occurs at or near the wellbore. The plugging problem is addressed by the present invention.